Shading elimination in electron multiplier pickup tube



www@ lw mfrrl m am SHADING ELIMINATION IN ELECTRON MULTIPLIER PICKUP TUBE Oct. 10, 1950 3S`A0d5`5'/ INVENTOR PAUL K. WE/MER ATTORNEY Patented Oct. 10, 1950 SHADING ELIMINATION IN ELECTBON MULTIPLlER PICKUP TUBE Paul K. Weimer, `Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 8, 1946,'Seria1No.,652,910

1o oiaims. 1

This invention relates to an improvement in television transmitters and more particularly, tol

an improvementin the method of operating a television pick-up or4 camera tube of the l'flpe.

known in the art as the Orthicon,wherein` electron multiplication for video signal intensification is provided within the tube.

In a television camera or pick-up tube of theY rent modulated by reason of the subtraction of electrons from the scanning beam by the target surface, and the current modulation represents a Source of video or image signals. It is frequently desirable to incorporate within the Orthicon, one or more stages of electron multiplication in order to increase the' video signal intensity so that the signal-to-noise ratio of the signal obtained from the OrthiconY tube may be materially improved.

In Orthicon tubes where the returning cathode ray beam traverses substantially the same path as the outgoing or scanning'cathode ray beam,

ther rst secondary electron emitter (i. e. the vfirst stage of the electron multiplier) is generally positioned relatively adjacent the electron gun structure, and, in some instances is a part of the beam forming electrode which is provided with a very small aperture (of the order of a few mils in diameter) in order to reduce the spot size of the scanning cathode ray beam. In spite of this location of the first secondary electron emitter, the returning beam does not continuously impinge upon the same particular area of the surface of this emitter, but, instead, moves about to a certain'extent in accordance with the horizontal and vertical deflections of the scanning beam. This movement of the return beam over the surface of the firstsecondary electron emitter produces undesired variations in the current intensity of the electrons derived from the emitter other than those brought about by reason of the variation in current intensity` of the returning cathode ray beam. vThis undesired variation is by reason of the fact that different portions of the surface of the secondaryv emitter are not absolutely uniform 'vor identical in emissionresponse and as the returning cathode-ray beam l moves slightly over this surface, undesired variaj tions in the amount of current Vobtained from this emitter occur. This undesired variation in the current obtained from the rst emitting vsurface of the multiplier produces an undesired dark spot in the resultant image and in previous circuit arrangements, this undesired dark spot has been substantially eliminated by dark spot compensation injection signals as is conventional practice where pick-up tubes of the so-called iconoscope type are used.

Rather than reduce the undesired dark spot to a negligible value by shading signal injection, it is preferable to effectively eliminate the production of the dark spot signal so that such compensation by shadingcontrol signal injection is unnecessary.

In the present invention, the undesired dark spot vthat is brought about by variations in the secondary electron emission characteristic of the surface of the first stage of the multiplier, is eliminated by obtaining from the electron multiplier video signals having a frequency materially higher than the frequency of occurrence of the frequency representing the undesired dark spot. It has been found that of the video signals available from the electron multiplier, if only those signals having a frequency higher than from kilocycles to 300 kilocycles are utilized, no appreciable or objectonal dark spot occurs since all Y signals having a frequency of the order corresponding to a dark spot are unused and eliminated. In order to supply that portion of the video signals having a frequency below 100 kilocycles to 300 kilocycles per second, a connection is made to the signal plate of the target electrode and video signals up to this range of frequencies are obtained from this electrode. The 10W frequency component of the video signals is, therefore, obtained from the signal plate of the target electrode whereas the high frequency component of the video signals is obtained from the electron multiplier of the Orthicon pick-up tube. Such an arrangement does not defeat the purpose of the electron multiplier since the decidedly improved signal-to-noise ratio may still be maintained. vThe low and high frequency components so obtained from the television camera or pick-up tube are then combined and in combining the signals it is necessary that they be combined in proper relative amplitudes and that the Phase relationship of the combined signals be proper in order that there will be present no phase distortion or phase shifting throughout the entire video spectrum. After the signals have been so combined, they are then applied to further video amplifiers and the signal is then used to modulate a radio frequency carrier after the necessary blanking, synchronizing and other pulses have been added, all in a manner well known to those skilled in the art, and thus not set forth herein in detail.

It is, therefore, one purpose of the presenti-nvention to provide a new and improved method of operating a television camera or pick-up ntube of the Orthicon type, including an electron multiplier, wherein any undesired dark spot signal is substantially completely eliminated.

Another purpose of the present invention resides in the use of a television camera or pick-up tube of the Orthicon type wherein an electron multiplier 'is provided such that substantially no. dark spot signal is present in the video signal output of the pick-up tube, and, accordingly, no'

dark spct'correction circuits areneeded.

' Still another purpose of the present invention resides in a new and improved method of andi means'for operating a television pick-up tube of the Orthioon type including an electron multiplier whereby video signals are-obtained both from the signal plate of the target electrode and from the collector electrode of the electron multiplier.'

A still further purpose of the present invention resides in an arrangement whereby video signals are obtained from both the target electrode and the electron multiplier in a television camera or pick-up tube ofthe Orthicon type and in which these outputs -Vsupply different portions of the video signal spectrum. 1

Still another purpose 'of' the present invention resides in the combining of the video'signal outputs from the signal plate of the target electrode and from the electron multiplier in a television camera tube of the Orthicon type in which the relativearnplitudeandV phase relationship of the combined signals are properly adjusted to produce the desired results. y y

Still other purposes and advantages will become more apparent to those skilled in the art', particularly from the following detailed description ,when considered in conjunction with the drawing wherein:

Figure 1V represents a preferred form of the present invention; and

Figure 2 represents curves used in explaining the operation of the system shown in Figure 1.

Referring now to the drawing and more particularly to Figure 1 thereof, a television pick-up or camera tube of theOrthicon type is shown at Ill. The tube includes an electron gun structure represented generally at I2 positioned in one end of the tube and a target electrode I4 positioned in the opposite end of the tube. The electron gun structure includes a cathode or electron emitter i6, a control electrode or grid I8 and an accelerating and beam forming electrode 2G. The end of the accelerating electrode is provided with a small beam defining aperture 22 havingY a diameter of the order of 2 or 3 mils through which electrons are permitted to pass. The relative potentials of the cathode and control electrode determine the current intensity of the produced cathode; ray beam, and electrons that pass through the aperture 22 are accelerated by the electrode 2li. The inside surface of the Orthicon` tube vis generally provided with a conductor surface 25 to which is applied a positive potential relative to the cathode of the electron gun structure but the potential is generally slightly negatif/e with respect tc the potential of the electrode 24 and the accelerating electrode 2i). Due to the accelerated force of the electrodes 28 and 2li, the cathode ray beam is projected through the tube at a predetermined velocity and this Velocity of electron motion is so controlled and finally so reduced that upon reaching the vicinity of the target electrode I4, it is substantially zero.

The targety electrode, in one form of the Orthicon tube, is constructed of a very thin wafer of mica or glass, one side (the scanned side) of which is provided with a light responsive surface while the opposite side is provided with a semiftransparent conducting film, generally referred to as thesignal plate. When a light image of an object 28 is projected upon the light responsive surface by a lens system, shown generally at 3U, the light causes the emission of photo-electrons from the photo-sensitized scanned side of the target surface. These emitted photo-electrons are collected by the conductingA surface 26 and by reason of their emission, a positive charge image is present on the mica or glass wafer corresponding in intensity variations to the light variations of the optical image.v Since the scanning cathode ray beam approaches the target electrode at substantially zero velocity, no secondary emission of electrons can be brought about by beam electrons impacting the target and consequently only such electrons are removed from the scanning beam as are necessary to cancel the positive charge image. The remaining electrons which constituted the scanning cathode ray beam are returned along substantially thev same path by reason of the increasing positive polarity of. electrodes 2S and 24 and these returningv `electrons'constitute a returning beam of electrons which is current modulated in accordance with the light image projected on the light responsive surface. The returning beam of electrons ultimately strikes the end surface 32 of the accelerator 20. The area struck by the returning beam of electrons; is small as compared with the end surface 32 but large as compared with the size of the defining aperture 22. This end surface is electron emissive and preferably has a high secondary electron emissive factor.

The Orthicon tube I0 is surrounded throughout a major portion of its length by a coil 3 which is the focusing coil and which is provided with direct current in order to produce an electromagnetic field which is in alignment with the axis of symmetry of the tube I0. This field is to maintain focus of the cathode ray beam. Between the focusing coil 34 and the tube proper there are additional coils 36 which are for defleetingI the cathode ray beam in the desired mutually perpendicular directions and the coils 36 are supplied with the necessary deflection current variations of line and field frequency. Accordingly, the combined field produced by the deflection coils and the focusing coil results in a continuously varying eld within the Orthicon tube. This combined field continuously varies in direction in accordance With the defiection currents in order to cause the cathode ray beam to scan a desired raster on the target surface.V

As the scanning operation takes place, video signals may be derived from the target electrode i4 by Way of conductor 38 much in the manner of operation of an Orthicon in 'which no electron multiplier is provided while the current variations of the returningcathode-ray beamalso provides a source of video signals. Since the end surface 32 of theY accelerating electrode 2l! is secondary electronemissive, the produced secondary electrons are directed upon successive multiplier stages 4i), 42, 44 and 46 of the electron multiplier. In each stage of the multiplier the current intensity is increased and secondary electrons emitted by the iinal surface or stage 45 are collected by the collector electrode 48 to which is connected conductor 55. Increased intensity video signals are,` therefore,` available from conductor 5B.

The electrodes of the electron gunstructure as well as the emitting s urfacespf the multiplier are maintained at proper relative potentials preferably by means of a voltage? divider represented generally at 52. vThefcoilector electrode 48 that is associated withthe final emittingsurfaceflle of the electron multiplierlis maintained at the most positive potential While the controlelectrodev of the electron `gun structure' is at the most negative potential. The final Ic'ollector electrodeiv -448 is, therefore, connected to a point near the positive end of the voltage divider'Z by a'load impedance 53. The collector electrode 48 is the output electrode of the multiplier.

It will be observe-.dA that a resistance 54 is included between-the potential divider 52 yand the control electrode i8 of the electron gun structure and furthermore, a resistance 56 is included between the cathode I6 andground potential (or an appropriate point along the voltage divider 52). The reason for including these resistances 54 and 56 will now be explained. i

As is well known to those l,skilled in the art, in the operation of a'v television `picli-vup tube of the Orthicon type blankingQhasjgenerally been accomplished by applying a "voltage variationvof rectangular waveform, to' the target` electrodek Vso that the target electrodeyis driven in a negative directionduring theintervals between successive lines and duringthe'intervals between successive frames. This blanking is"desirablefinjorder to remove from thevideo signal seriesany trash or signal that would `,be generated during the fly-back time; In the( present invention such a form of blanking would vnot be directly feasible since the blanking signalwould bel undesirably injected into the portion' of the television video signal that is obtained from conductor 38. Accordingly, in the present invention, blanking is accomplished byV driving both the control electrode and cathode of the electron gun structure in a positive direction during the interval between lines and between frames. VBoth the cathode and the control electrode are so modulated in order that the current intensityvof the produced cathode ray beam will not be materially affected. Furthermore, the extent towhich the cathode and control electrode are drivenin a positive direction during these intervals is `only such as will reducethe velocity of the cathode ray beam to a point such that none of the electrons constituting the beam will besupplied to the target electrode. It is desirable that thecurrent intensity ofthe cathode ray beam generated by the gun structure be maintained as constant as is possible in order to avoid undesired iluctuations in the video signal output from conductor 5i] while on the other hand it'is desirable that noY electrons constituting thebeam be supplied to the target electrode during'the blanking intervals in order that a proper black level can be established directly from the produced video signals. Accordingly, a voltage variation ofA substantially rectangular waveform, such as shown at 5T, is applied4 between ground (or a point of fixed potential) and the cathode i6 and this same voltage variation is also applied to the control electrode I8 by way of condenser 53.

The video signals obtained from conductor 5i) (through the direct current isolating condenser 5 l at the output of the electron multiplier portion of the Orthicon contains the entire video signal spectrum together with undesired signals or modulations representing the dark spot referred to above. As previously stated, this dark spot is produced `by reason of the fact that the returning beam of electrons actually scans over a small area of the end surface 32 of the accelerating electrode 20 and since this small scanned area does not have a uniform secondary emission ratio, the current intensity derived from the surface 32 fluctuates not only in accordance with the curment intensity of the returning beam of electrons but also (slowly) in accordance with slight variations in secondary electron emission over the small scanned area of the surface 32. The signals obtained from conductor 50 are then applied to a high-pass lter 60 where video signals below approximately 200 kilocycles per second see, for instance, Fig. 2) areattenuated while-signals above 200 kilocycles per second are permitted to pass. Signals that are derived from conductor 38 are applied to a low-pass lter 62, which is complementary to the high-pass lter 58. The low-pass lter, for instance, passes frequencies up to the order of 200 kilocycles per second but attenuates frequencies above this order.

Characteristics of the filters 6@ and 62 are represented in Figure 2 whereV curve GEA represents, for example, the high-pass characters of the nlter iwhile curve 62A represents the characteristics of the complementary low-pass lter 52. The particular frequency of the cut-01T region of the two filters is immaterial insofar as this invention is concerned and need not exactly conform to the curves shown in Figure 2 so long as their characteristics are complementary in order that a satisfactory combination of the signals is possible. When a cut-off frequency 'of the order of 200 kilocycles per second is chosen, however, all shading having a coarser resolution than 5% of a television line would be effectively eliminated. Virtually all of the objectionable shading is well within this range and in fact is represented by,

frequencies of a much lower order.

Since the video signals obtained from conductor -50 have passed through an electron multiplier,

their intensities are increased as compared with the intensity of the'signals available from the conductor 38. Accordingly, it is then necessary to provide an amplifier 64 for amplifying the low frequency portion of the video signals in order that 'their amplitudes will be comparable with the amplitudesof the high frequency portion available from the high-pass lter Si). The amplier 64 should be so designed as to match, in performance, the electron multiplier so that when the outputs from the high-pass filter 59 and the amplifier 64 are combined into a single video signal spectrum Vsubstantially no phase distortion will be present throughout the entire sped trum. These combined signals are then available at conductor 66 and they may be applied to a further video signal amplier 68`where they are absence madev available in increased intensity at the output conductor 10.

By reason of the above arrangement, it will be apparent, therefore, that variations in gain over the surface of the rst stage of the electron multiplier included in a television camera or pick-up tube of the Orthicon type may be substantially eliminated rather than being transmitted as an undesired shading in the video signal. Shading controls may, therefore, be completely eliminated. The extent of the undesired dark spot is a function of the particular tube since, as stated above, returning electrons do not spray uniformly over the rst stage of the multiplier, but, instead, scan a small portion of the surface in a fairly well focused beam. If this small portion of the surface has substantially uniform secondary electron emission characteristics, the produced dark spot signal will be small, however, if the varia-- tion should be relatively large, the dark spot signal would be correspondingly large.

Although the present invention is described and shown as adapted to one form of an Orthicon tube shown in Figure 1, the same method and circuit arrangement is equally applicable to any Orthicon type tube wherein an electron multiplication is employed. The invention is also applicable to Orthicons of the type including image intensification. In some forms of Orthicon tubes, a screen electrode is maintained closely adjacent the scanned side of the target electrode and the position of this screen tends to reduce the area scanned on the surface of the rst stage of the multiplier by the return beam. The present invention described herein would be particularly effective in such a tube in removing any'undesired dark spot since the effect of the use of such a screen would then reduce the area scanned. on the rst emitter`on the multiplier and to thereby shift the frequency representing the undesired dark spot to a lower frequency range. Any shading introduced by low frequency Variations in transmission of the screen would likewise be eliminated by the present invention.

Having now described the invention, the following is claimed:

l. A television transmitter including a television camera or pickup tube of the type including a target electrode and wherein a low velocity cathode ray beam is developed and appropriately deflected to scan the target electrode 'and wherein an electron multiplier is adapted to receive and increase in current intensity the electrons constituting the current modulated and returned stream of electrons of the scanning beam comprising means for deriving television video signals from the target electrode of the pick-up tube, means for deriving television video signals from the electron multiplier, a low-pass filter associated with said rst named source of television video signals, a high-pass filter associated with said second source of television video signals, means for adjusting the relative intensities of the signals passed by each of said filters, and means for combining the television video signals passed by each of said filters so that the combined signals constitute a composite and complete television video signal series.

2. A television transmitter including a television pick-up tube of the type having a target electrode, an electron gun structure for generating a low velocity cathode ray beam adapted to scan the target electrode and an electron multiplier adapted to receive the electrons constituting the returned scanningy beam comprising means for deriving video signals from the target electrode, means for also deriving video signals from the electron multiplier, a low-pass filter associated with said rst named source of video signals, a high-pass filter associated with said second source of video signals, means for altering the relative intensities of the signals passed by each of said filters sol that they are of substantially identical intensity, and means for combining the video signals passed by each of said filters so that the combined signals constitute a complete video signal series.

3. A circuit arrangement such as defined in claim 2 which includes, in addition, means for simultaneously applying pulses of positive polarity to the control electrode and cathode of the electron gun structure of the television pick-up tube in order to produce blanking during selected portions of the scanning cycle.

4. A television transmitter including a televisionv pick-up tube of the type having a, target electrode whereupon an electrostatic charge image representative of an optical image isI adapted to be developed, an electron gun structure in said tube adapted t0 generate a, beam of electrons which may be directed toward the target electrode at low velocity and means suitably to defleet the said beam in order to scan the target electrode thereby to produce television video signals at the target, the scanning beam of electrons being returned through the tube and applied to an electron multiplier for increasing and amplifying the current intensity of the returned cathode ray beam whereby television video signals are also present at the collector electrode of the electron multiplier, comprising a high-pass lter adapted to receive television video signals from the collector electrode of the. electron multiplier portion of the pick-up tube and to transmit the portion of these video signals having a frequency greater than a predetermined value, a low-pass filter associated with the target electrode and adapted to receive television video signals from the target electrode whereby the low-pass filter will transmit the portion of these video signals having a frequency less than the said predetermined value, means to amplify the signals passed by at least one of the filters so that the signals are of comparable amplitude, and means to combine the signals in order to produce a single composite and complete television video signal series.

5. A television transmitter including a television pick-up tube of the type having a target electrode upon which an electrostatic charge image is adapted to be developed when light images are directed upon the camera tube, an electron gun structure in said tube adapted to generate a beam of electrons which may be directed toward the target electrode at low velocity in order to scan the target electrode to produce thereat video signals, and an electron multiplier for receiving the returned cathode ray beam whereby video signals are also present at the collector electrode of the electron multiplier, comprising a high-pass circuit adapted to receive video signals from the collector electrode of the electron multiplier portion of the pick-up tube and to transmit video signals above a predetermined frequency, a low-pass circuit adapted to receive video signals from the target electrode and to transmit. video signals below said predetermined frequency, means to adjust the video signals passed by the circuits so that the signals are of comparable amplitude, and means to combine the signals in order to produce a video signal series including both of said selected frequency images.

6. A circuit arrangement such as dened in claim which includes, in addition, means for simultaneously applying pulses of positive polarity to the control electrode and cathode of the electron gun structure of the television pick-up tube in order to produce blanking during certain portions of the scanning cycle.

7. A television transmitter including a television pick-up tube of the Orthicon type wherein is included a target electrode, means for generating a low-velocity beam 0f electrons and an electron multiplier so that when the electron beam is caused to Scan the target electrode television video signals are available from both the target electrode and from the electron multiplier, comprising an ampililer connected to respond to the video signals at the target and to increase the intensity of the television video signals as derived from the target electrode so that the signals so amplified are comparable in intensity with the signals derived from the electron multiplier, a low-pass lter associated with the signals availa-ble from the target electrode, and a high-pass lter associated with the signals available from the electron multiplier, the characteristic of the lters being such that when the outputs from each of the filters are combined a single television Video signal series may be produced.

8. A television transmitter including a television pick-up tube of lthe Orthicon type which includes a target electrode, an electron gun Structure adapted to generate a low-velocity beam of electrons and an electron multiplier having an.

output electrode and adapted to amplify the current intensity of the returned beam of electrons, comprising a rst circuit arrangement capable of passing all frequencies below approximately 20'0 kilocycles per second associated with the target electrode and adapted to receive television video signals available from this electrode, a second circuit arrangement capable of passing all frequencies above approximately 200 kilocycles per second associated with the output electrode of the multiplier and adapted to receive television Video signals from the multiplier, means to amplify the signals passed by said first circuit arrangement so that the intensity of the signals passed by each of said circuit arrangements may be substantially identical in intensity, and means for combining the television video signals passed by each of said circuit arrangements whereby a single complete television Video signal series may be produced.

9. An apparatus such as defined in claim 8 which includes, in addition, means for momentarily altering the potential of the control electrode and the cathode of the electron gun structure in a positive direction for predetermined short intervals during the target electrode scanning cycle.

10. A television transmitter including a television pick-up tube of the Orthicon type which includes a target electrode, an electron gun Structure adapted to generate a, low-velocity beam of electrons and an electron multiplier, comprising a rst circuit arrangement associated with the target electrode and adapted to pass all video signals below a, predetermined frequency when television video signals from the target electrode are supplied thereto, a second circuit arrangement associated with the electron multiplier and adapted to pass all video signals above the predetermined frequency when television video signals from the multiplier are supplied thereto, means to amplify the signals passed by one of said circuit arrangements so that the intensity REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 2,182,578 Blumlein Dec. 5, 1939 2,250,293 Bunger July 22, 1941 

